Field of the Invention
This invention relates to video signal clamping.
FIG. 1 of the accompanying drawings shows part of a video signal waveform in the region of a horizontal blanking interval thereof. During the horizontal blanking interval, the waveform should be steady at a level which is the black level of the picture, except during the occurrence of the line synchronization pulse (which goes to a blacker-than-black level). It is well known in the art to clamp the black level to a reference black level. The clamping performs two functions. The first is to remove any spurious signal ("hum" or noise), e.g. in the form of a distorted supply or mains waveform (e.g. 60 Hz or 50 Hz) that may be modulated on the black level. This must be removed to avoid gross distortion of the picture. The second function is one of d c restoration, namely to clamp the black level to a reference level so that it does not vary with picture content.
Conventionally, analog circuits have been used to perform video signal clamping. As is well known, however, analog circuits tend to be unstable with temperature. So, it has been previously proposed to supplement an analog clamping circuit with a digital clamping circuit. According to this proposal, the analog circuit performs the above-described function of removing the distorted supply waveform ("supply hum") and the digital circuit follows up to provide the above described function of dc restoration, that is to say removing dc imbalance. The previously proposed digital circuit employed an analog-to-digital converter (ADC) which, using the leading edge of the line synchronization pulse as a time reference, took and digitized a sample of the level of the signal at a position on the "back porch" of the horizontal blanking interval, namely that part of the waveform that extends from the trailing edge of the line synchronization pulse to the start of the picture signal. The digital sample was compared with a reference level in a digital comparator and the resultant digital error signal was supplied to an analog integrator having a long time constant (of the order of seconds) to produce an analog error signal (integrated over many lines of the picture) which was combined with the original video signal in a sense to compensate for any error in the black level. Thus, the previously proposed digital circuit took one sample of the signal (on the back porch) per line and relied on a very long time of integration. It performed dc restoration only, the analog clamping circuit providing the function of removing spurious "supply hum".
An object of this invention is to provide a video signal clamping circuit operating in the digital domain which can provide dc restoration and remove "supply hum".